1. Field of the Invention
The present invention relates to a water addition apparatus that adds water or an aqueous solution to a sheet in order to suppress wavy edge or curling of the sheet and an electrophotographic image forming apparatus provided with the water addition apparatus.
2. Description of the Related Art
Conventionally, in an electrophotographic image forming apparatus, an image (hereinafter referred to as a toner image) is visualized such that a development device develops an electrostatic latent image formed on a photosensitive drum that is of an image bearing member. The toner image is transferred to the sheet by an electrostatic force, and the toner image transferred to the sheet is heated and pressurized by a fixing device so that the image is fixed to the sheet, thereby forming an image on the sheet.
Frequently a thermal roller fixing type is adopted in the fixing device of the image forming apparatus. In the thermal roller fixing type fixing device, a fixing roller that includes a built-in heater and a pressure roller that is rotatably in contact with the fixing roller with a pressure constitute a fixing nip, and the sheet to which the toner image is transferred passes through the fixing nip to fix the toner image to the sheet.
In the thermal roller fixing type fixing device, the sheet is introduced to the fixing nip, which is formed by rotatably bringing the pressure roller including an elastic layer made of rubber into contact with the fixing roller, and the sheet is conveyed while nipped between the pressure roller and the fixing roller. At this point, a halogen heater is used as a heat source to heat the fixing roller, and the fixing roller is maintained at a predetermined temperature. Therefore, the unfixed toner image is thermally fixed to a surface of the sheet.
Through the fixing process, because the heat and the pressure are substantially simultaneously applied onto the sheet to which the toner image is transferred, water in the sheet is evaporated while the sheet is pressurized.
At this point, a phenomenon called “curling” in which the sheet is curved or a phenomenon called “wavy edge” in which the sheet is wavy is generated by a change in water amount in the sheet and a stress applied to the sheet.
Assume that most commonly used sheet-like paper is observed herein in terms of fiber. The paper is constructed by short fibers that are tangled up, and the water is present in or between the fibers. Smoothness of the paper is maintained because the fibers and the water are in an equilibrium state while a hydrogen bonding is generated.
However, when the heat and the pressure are substantially simultaneously applied to the paper in the fixing process, the fibers deviate from one another due to the pressure. At this point, when the paper is heated to evaporate the water, the hydrogen bonding is further generated between the fibers, resulting in a deformation of the paper. When the paper is left, the paper absorbs moisture from the surroundings, and the hydrogen bonding between the fibers is cut off again to return to the original state. However, the water does not invade in some fibers, and the deformation of the paper remains. A pattern of the deformation includes the curling and the wavy edges; the curling is generated by a difference in expansion and contraction between a surface and a backside of the paper, and the wavy edges are generated by the difference in expansion and contraction between a central portion and an end portion of the paper.
U.S. Pat. No. 5,850,589 discloses a solution to the problem. U.S. Pat. No. 5,850,589 describes an apparatus and a system, which are used to prevent the curling and the edge wavy edges caused by run-out of the water from the sheet in the fixing process of the electrophotographic image forming apparatus.
The apparatus described in U.S. Pat. No. 5,850,589 adds a controlled amount of water to a single side or double sides of the sheet. The apparatuses are disposed in both the surfaces of the sheet, and include a water jet having a reservoir in which a liquid is stored and a pair of pressure rollers having a cylindrical outer surface. The pair of pressure rollers is aligned along axes thereof such that a nip is formed between the cylindrical outer surfaces. The apparatus also includes a control device that controls the water added from the water jet to a selected portion of each passing sheet before the sheet enters the nip formed between the cylindrical outer surfaces.
In the case that the paper is used as the sheet, the water is added to the paper in which the water has been deprived by the heat and the pressure through the fixing process, whereby the lost water is supplied to reduce the curling and the wavy edges. As the amount of water in the paper is increased, a Young's modulus of the paper is decreased, stiffness of the paper then is decreased, and therefore the wavy edge is reduced. When the water invades in a gap between fibers, the hydrogen bonding between the fibers is cut off again by the fixing device, thereby reducing the wavy edge.
Recently, more image forming apparatuses with a higher speed are introduced in order to implement high productivity of the image forming apparatus, and there is a demand to enhance a recording sheet conveying speed. However, because a rate at which the water invades in the sheet is kept constant, a time during which the sheet passes through the water supply device becomes shortened as the recording sheet conveying speed is enhanced. Therefore, unfortunately the amount of water necessary to reduce the curling and the wavy edge cannot be supplied to the sheet.
An object of the present invention is to provide an image forming apparatus that can supply the water necessary to reduce the curling and the wavy edge to the sheet even when the sheet conveying speed is enhanced.